Why do we "warmup" IP addresses

IP address warmup is a big issue for anyone moving to a new IP address for sending.
I’m constantly being asked how to warm up an IP. My answer is always the same. There’s no right way to warm up an IP nor is there a specific formula that everyone should follow.
What warming up is about is introducing mail traffic to receiving spam filters in a way that lets the filter know this is a legitimate email stream. This means sending small but regular amounts of mail that recipients interact with. As the filters adjust to the amount of mail from that IP, more mail can be sent over that IP. Increase the mail volume over the next few weeks until the desired volume is reached.
There are a couple things to remember about warming up.

• Warming up is about the regular volume from an IP. IPs cannot be “pre warmed” and then left without traffic. Stop mailing from that IP and lose any benefit gained.
• Warming an IP will not compensate for poor practices. You can only warm an IP to the reputation of the mail sent from that IP.
• Warming creates a reputation, but that reputation is not permanent. Changing mail sent over the IP can increase or decrease the reputation of the IP.

So what’s the best way to warm up an IP? There really isn’t a best way. Each IP warms differently. There are some guidelines, however,

• Send first to your best, most active customers.
• Send consistently.
• Start with a hundred or so messages an hour.
• Increase the hourly rate gradually.
• Monitor your logs.
• If the ISPs start sending back 4xx failures, you’re going too fast so slow down.
• Don’t try to start a new IP and a brand new domain at the same time.